Capacità della veccia (Vicia villosa Roth) di assorbire ed assimilare il pool di P dalle radici di riso ricoperte da placche di Fe

Legume cover crops (LCC) are often included between two rice cropping seasons to enhance the nitrogen (N) soil availability by their roots’ symbiosis with nitrogen-fixing bacteria. LCC can also act as catch crops by acquiring important amounts of phosphorous (P). Due to their high P requirements, legumes have developed P acquisition strategies in P deficiency conditions: extension of the roots (root foraging) and mobilization of phosphate anions from organic and inorganic poorly soluble P sources (P mining) through the exudation of protons, organic acids and enzymes. During the rice cropping season substantial amount of P is mobilized under reducing soil conditions. This can be retained within the iron (Fe) plaques deposited on the rice roots. However, little is known about the fate of the nutrients after rice harvesting. We hypothesize that LCC could adopt strategies to acquire P from the Fe plaques, which are in close proximity to their roots after the rice harvest. The aims of this thesis were to investigate (i) whether hairy vetch plants (Vicia villosa Roth) are able to access this P pool, and (ii) the mechanism governing P release. In a P-free quartz sand substrate two plants were grown with different P sources: high (+P +Fe) and low (–P +Fe) P adsorbed on Fe plaque, high (+Pi) and low (-Pi) inorganic P, and the control with rice roots cleaned from Fe plaque (–P –Fe). At 45 days after seeding (DAS) two thirds of the plants were treated with labelled carbon dioxide (13CO2), to trace carbon (C) fluxes within the plant and the roots’ C exudations. The other third of the plants was used as untreated control. The vetch plants were harvested in two sampling times: one third 45 DAS and two thirds (half labelled and half not) 60 DAS. The 13C content in the plants of the first harvest time was used to determine the total amount of fixed 13C and used as reference for evaluating the subsequent allocations of fixed 13C between shoots, roots and Fe plaque. The root release of H+ was evaluated by measuring the pH of the rhizosphere. The δ13C of rice roots was determined to verify the transfer of vetch photosynthates to rice detritusphere by root C exudation, as well as the content of Fe and P in the remaining plaque. The vetch capability to mobilize and to take up the P adsorbed onto Fe plaques was significantly influenced by the P availability. In this regard, the plants of the treatment with low P availability (–P +Fe) showed a more active plant response than the ones of the high P treatment (+P +Fe) in terms of a larger C allocation to the roots and a greater root C exudation. Moreover, only the –P +Fe treatment displayed a higher acidification of the rhizosphere compared to the control (–P –Fe). Nevertheless, the Fe plaque of the treatment with high P availability (+P +Fe) showed a P desorption and Fe dissolution significantly larger than the ones of –P +Fe treatment, resulting also in a significantly higher plant P uptake. However, vetch plants belonging to both the treatments (+P +Fe and –P +Fe) have shown a response to P deficiency, in terms of photosynthetic C allocation to the roots and to Fe plaque. The tested capability of the hairy vetch to sense the P on the plaques and to exude C in order to mobilize it, could make the introduction of this legume in rice cropping system important not only in terms of increased nitrogen use efficiency but also to mobilize a P pool that would otherwise not be available for the rice uptake.